(a) Field of the Invention
The present invention relates to a process for directly preparing glycine from glycolonitrile via hydantoin, and more specifically it relates to a process for obtaining glycine by the reaction of glycolonitrile, ammonia and carbon dioxide.
(b) Description of the Related Art
Glycine is a useful compound which is widely used as a raw material for food additives for processed foodstuffs, agricultural chemicals and medicines.
Heretofore, as methods for manufacturing glycine, there have been mainly known an amination method using monochloroacetic acid, a Strecker method, a hydantoin method and the like.
The Strecker method comprises reacting ammonia with glycolonitrile which can be synthesized substantially quantitatively from hydrocyanic acid and formalin, carrying out hydrolysis with an alkali such as NaOH to form a metal salt of glycine, and then removing the metal with an acid such as sulfuric acid to prepare free glycine. In this method, the separation of a neutral salt such as sodium sulfate is necessary after the hydrolysis, and the treatment of this salt impairs economy substantially.
On the other hand, the above-mentioned hydantoin method comprises reacting glycolonitrile, which can be synthesized from hydrocyanic acid and formaldehyde, with ammonia and carbon dioxide gas in the presence of water to form hydantoin, and then hydrolyzing the same to prepare glycine When an alkali such as NaOH is used in the hydrolysis, the separation and treatment of sodium sulfate are necessary, as in the Strecker method. In this hydantoin method, there is a process for directly obtaining free glycine without using an alkali such as sodium hydroxide (hereinafter referred to as "the hydantoin-via direct method"), and this hydantoin-via direct method is an economical process for the preparation of glycine without a by-product such as sodium sulfate and without environmental pollution.
As some examples of such a hydantoin-via direct method, there have been known a method in which hydrogen cyanide, aldehyde, ammonia and carbon dioxide are heated at 100.degree. C. or more in an aqueous medium (U.S. Pat. No. 3,536,726), a method in which hydrogen cyanide, formaldehyde and ammonium carbonate are heated in an aqueous medium (Japanese Patent Laid-open No. 49-127915), a method in which glycollonitrile, ammonia and carbon dioxide are heated to remove ammonia and carbon dioxide, followed by a treatment with an alkylamine or an alkylammonium hydroxide, and a method in which glycolonitrile, ammonia and carbon dioxide are heated to remove ammonia and carbon dioxide, followed by a treatment with a mineral acid to effect hydrolysis (Japanese Patent Laid-open No. 53-28116). In U.S. Pat. No. 3,538,726, it is also disclosed that most of the glycine is separated from a reaction solution, and the remaining solution (hereinafter referred to simply as "mother liquor") is returned to a reaction zone.
However, these methods mainly intend to improve the yield of each reaction, and they do not provide any techniques for industrially manufacturing glycine inclusive of the separation manner of ammonia and carbon dioxide, the concentration manner of the reaction solution and the recycling manner of the ammonia, carbon dioxide and mother liquor.
According to the investigation by the present inventors, the following has been found: In the step of concentrating the reaction solution obtained by the hydantoin-via direct method, to isolate glycine from it, a normal heating treatment for concentration of it makes the isolation of glycine impossible. Additionally, at this time, by-products such as 2,5-diketopiperazine having extremely low solubility in water increase perceptibly, so that it is practically impossible to isolate glycine from these kinds of by-products.
Furthermore, the present inventors have found that the employment of glycolonitrile containing water (or an aqueous glycolonitrile solution) prevents the decomposition of glycolonitrile and a side reaction by which a colored material is formed. The reaction of the present invention stoichiometrically produces 1 mole of glycine from 1 mole of glycollonitrile and 1 mole of water, and therefore it consumes water, but the amount of water which is consumed by the reaction is small. Therefore, when the water-containing glycolonitrile (or the aqueous glycolonitrile solution) is used which is a preferable feed morphology in that the decomposition of the glycolonitrile is prevented, water corresponding to the contained water therein is excessive, so that it is accumulated in the system. That is, it is required to purge water corresponding to the excess water from the system. However, the method for purging water containing the carbon dioxide and ammonia or the mother liquor from the system is not economical, and such a purge of the carbon dioxide and ammonia or organic materials in the mother liquor from the system is also undesirable from the viewpoint of environmental pollution. Therefore, the present inventors have found that a process for preparing high-purity glycine in a high yield substantially without discarding ammonia and the carbon dioxide or the organic materials is essential to industrially manufacture glycine.
The present inventors have intensively conducted research on a process for preparing glycine in a high yield by concentrating a reaction solution effectively in accordance with the hydantoin-via direct method, and as a result, the present invention was made.